This invention relates to a method of bonding a tubular shaft to an end fitting to produce a joint which assures axial alignment of the shaft and the end fitting and is capable of transmitting a high torque.
One obvious method of bonding a tubular shaft to an end fitting is to provide the end fitting with a rod-shaped mating section which fits into the shaft, coat the mating section and/or the inner circumference of the shaft with a suitable cement or bonding agent, and then insert the mating section into the shaft. However, this method suffers from the disadvantage that the bonding agent employed tends to rub off the mating section when it is inserted into the shaft if too tight a fit is provided, or, alternatively, is accompanied by leakage of the bonding agent from the annular space between the mating section and the shaft if the fit is too loose. In any event, the end result is a loss of bonding agent and a weakening of the joint, so that the desired degree of bonding is not attained. Furthermore, in certain instances, as in the case of the bonding of a universal joint fitting in a drive shaft, it is essential that the fitting be aligned concentrically in the shaft, and this method does not provide for obtaining a secure joint having such concentric alignment.
An alternative method of bonding a tubular shaft to an end fitting having a rod-shaped mating section is to first insert the mating section into the shaft, and then inject the bonding agent into the annular space between the mating section and the shaft from a hole in the side of the shaft. A second hold in the side of the shaft is also necessary in such case, of course, to allow for the release of air displaced by the bonding agent. However, not only is this technique quite slow, but oftentimes it fails to completely fill the annular space between the mating section and the shaft with bonding agent, resulting in voids between them and a poor bond. Visual inspection of the joint, of course, is not possible, so that the presence of such voids, and the resulting poor bond, goes undetected.
One very effective way of bonding a tubular shaft to an end fitting having a rod-shaped mating section is disclosed in copending application Ser. No. 676,866, now U.S. Pat. No. 4,041,599. Said method comprises: (1) fitting a pair of parallel annular grooves positioned near the base and top of the mating section of the end fitting with a pair of resilient sealing rings having inner diameters such that they are retained in place by the grooves and outer diameters greater than that of the mating section, said sealing rings being designed to fit snugly into the tubular shaft and form a tight fit between the mating section and the shaft; (2) inserting the base of the mating section containing the first of said resilient sealing rings into one end of the tubular shaft so as to compress said resilient sealing ring snugly between the mating section and the inner circumference of that end of said tubular shaft, said end of said tubular shaft having the neck of a funnel tightly fitted about its outer circumference; (3) filling the funnel with a suitable liquid bonding agent; (4) further inserting the mating section of the end fitting into the tubular shaft, thereby creating a vacuum in the annular spacing between said mating section and said tubular shaft so as to cause the bonding agent in the funnel to flow into and fill said annular spacing; (5) continuing to insert the mating section of the end fitting into the tubular shaft until the annular space between said mating section and said tubular shaft has been completely filled with bonding agent and the second resilient sealing ring at the top of the mating section has been forced into the shaft and snugly compressed between said mating section and said tubular shaft, thereby entrapping and sealing said bonding agent in said annular space, and ( 6) curing said bonding agent to the thermoset state so as to bond the shaft and the mating section of the end fitting together. Not only does this method provide a secure bond between the mating section of the end fitting and the shaft, but it also assures accurate centering of the mating section in the shaft. For this reason it is particularly suitable whenever concentric alignment of the mating section of an end fitting in a shaft is necessary, such as when the mating section of a universal joint fitting is bonded to a drive shaft. However, while the joint produced in this manner has been found satisfactory for most applications and capable of transmitting relatively high torque loads, it has not been found satisfactory for transmitting extremely high torque loads. Thus, although fully acceptable as a bond between a universal joint fitting and a drive shaft when such assembly is used in automobiles, the joint produced in this manner has been found wanting when such assembly is used in trucks.